1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly to a method of manufacturing a gallium nitride (GaN) based semiconductor light emitting device, which ensures a high brightness while lowering a contact resistance near a surface formed with a p-side electrode.
2. Description of the Related Art
In general, a gallium nitride-based semiconductor is used for a light emitting diode for emitting light in a wavelength range of a blue light or a green light and usually comprises semiconductor materials with the formula AlxInyGa(l-x-y)N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1). As for the light emitting device generating a green light, a gallium nitride (GaN) semiconductor material with a large energy band gap of about 3.4 eV is usually used.
As the GaN-based semiconductor generally has a large energy band gap, there is a difficulty in achieving an ohmic contact with an electrode thereon. Specifically, a p-type clad layer to be formed with a p-side electrode has a large contact resistance, causing a problem of raising operating voltage of the device and amount of heat.
Conventionally, as for a method for overcoming the problem for the ohmic contact, various methods have been suggested. However, since an ohmic contact portion is provided as a primary light emitting plane, it is needed to satisfy a requirement of ensuring a transmission of light generated from an active layer through the light emitting plane. Thus, only a few methods could be restrictively adopted in practice as a method for lowering the contact resistance.
As a conventional method, U.S. Pat. No. 5,563,422 entitled “Nitride gallium-based III–IV compound semiconductor device and method of producing the same,” which is assigned to Nichia Chemical Industry Limited, discloses a transparent electrode using a bi-layer of Ni-Au. FIG. 1 shows a light emitting device according to an embodiment of U.S. Pat. No. 5,563,422.
As shown in FIG. 1, a gallium nitride-based semiconductor light emitting device 10 of the prior art comprises an n-type GaN semiconductor layer 13 formed on the substrate 11, a GaN/InGaN active layer 15 having a multi-well structure and a p-type GaN semiconductor layer 17. An n-side electrode 19a is formed on the n-type GaN semiconductor layer 13, which has a portion of the p-type GaN semiconductor layer 17 and GaN/InGaN active layer 15 removed. A transparent electrode 18 and a p-side electrode 19b are sequentially laminated with Ni and Au on the p-type GaN semiconductor layer 17. The transparent electrode 18 is a translucent layer for improving the contact resistance and forms the ohmic contact through a deposition process for Ni/Au and a subsequent heat treatment.
It is desirable that the transparent electrode 18 is formed as thick as possible in order to improve current injection efficiency. However, as the transparent electrode 18 is formed of metals, it could be difficult for the transparent electrode to have a desired translucency. Even though a NiO layer of a relatively high translucency is formed through the heat treatment, it has a transmittance of merely 60%, so that light emitting efficiency can be deteriorated as the transparent electrode thickens.
Thus, the transparent electrode should be limited to a thickness of 100 μm or less, causing a restriction in improving the current injection efficiency.
Further, even though the transparent electrode of the metals has a proper thickness in consideration of a desired transmittance, it inevitably encounters some reduction in brightness due to a restrictive transmittance in the transparent electrode that is made of the metals having a high photon absorption rate, such as Au.